Arch shape cross section corrugated chambers that are made of thermoplastic have been widely used for leaching of water into the earth in recent years. For example, they have been used in connection with septic systems and for receiving and dispersing storm waters. Some of such chambers are made by thermoforming of sheet. The better chambers are made by injection molding of thermoplastic, and, they have had internal and external ribs. The ribs provided strength, to resist the forces from soil, or from vehicles on the soil surface, during use.
The strengthening ribs of chambers serve the additional purpose of providing thicker sections in the injection mold cavity, through which melted plastic from sprues, i.e., injection molding nozzle locations. When gas assist injection molding is used, gas is injected at the sprues and at other locations; and gas flows with plastic along the ribs. Thus, a side benefit of ribs is that they provide channels in which plastic flows, to fill the mold. Ribs supplement, or can in part eliminate the need for, runners and thickened cross sections, which are needed for obtain a properly filled mold. U.S. Pat. No. 5,401,459 of Nichols et al. describes some of problems and solutions associated with obtaining proper flow distribution in a molded chamber.
When such kinds of chambers are shipped from the factory to distributors and to points of use, they are nested. The number of chambers which can be carried by a vehicle is a function of the cargo volume, rather than weight. Thus, how closely or densely the chambers nest becomes of economic interest with respect to shipping costs. Heretofore, the necessity of having ribs for strength limited the density of nesting. Thus, if localized chamber wall thickness increases were necessary to provide plastic flow channels, they could be included since nesting height was not adversely affected.
Some newer chamber designs are shown in commonly owned U.S. patent application Ser. Nos. 10/402,414 filed May 4, 2001, No. 09/849,768 filed May 28, 2003, both of Kruger et al., and in No. 10/677,938 of Brochu et al., filed Oct. 1, 2003. The improved design enables a corrugated chamber which does not need strengthening ribs and has better nesting than the old chambers. The strength of the improved engineered design chambers arises from the shape of the corrugations and the curve of the arch shape cross section, as described in the referenced applications. Furthermore, some of the newer chambers have thinner walls, of the order of 0.090 inch compared to around 0.150 inch in older chambers. The newer designs provide substantially improved nesting.
The absence of ribs and the thin walls in the new design chambers can create special manufacturing problems with respect to plastic flow and injection molding, which must be overcome to obtain a sound product. To obtain good fill of the mold in such circumstances, a good number of closely spaced sprues can be used, as can substantially increased injection pressure. However, often product design, and mold and machine factors, urge in the opposite direction. In another approach, the chamber design can be altered, so the wall thickness is increased locally to create flow channels for plastic. However, when the latter approach is employed, particularly when the need is to get good feeding along the length of a chamber, there can be an adverse effect on nesting height as detailed further in the description which follows. Thus, there is a need for new approaches inchamber design or manufacturing method, which improve melted plastic flow but do not adversely affect nesting height.